1. The Field of the Invention
This invention relates to road construction and, more particularly, to novel systems and methods for spreading loose aggregate onto a road surface.
2. The Background Art
Since the cost of roadways is substantial, it is desirable to lengthen the useful life of a road as much as possible. To make roads last longer, they are sometimes treated for preventive maintenance. Preventive maintenance of a road surface can reduce the likelihood of the road surface becoming cracked or chipped, having potholes appear, and developing other similar problems that often occur with roads.
There are several ways that road surfaces can be treated to help lengthen the life of the roadway. Chip seals, also known as oil and screenings, aggregate seal coats, and armor coats, are surface treatments which are placed on an existing asphalt pavement. Applying a chip seal to a road is one form of preventive maintenance that can be used to increase the life of a roadway. Generally, the chip seals do not add structural strength to the roadway, but they do produce an ideal all-weather surface, renew weathered pavement, improve skid resistance, and seal the old pavement.
Chip seals are applied to a roadway by first spraying the pavement with a binder, often an asphalt emulsion, from an oil distributor truck. This binder is a tacky coating placed onto the road surface that acts to bind gravel, to be applied soon thereafter, to the road surface. After the binder is applied to the road surface, a uniform application of cover aggregate (similar to and including fine gravel) is applied, usually by a self-propelled chip spreader. For example, chip seals usually employ 1/4 to 1/2 inch (0.64 to 1.3 cm) aggregate. As the aggregate (i.e., gravel) contacts the sticky binder coat, it tends to stick to the road surface. The aggregate is usually rolled as soon as possible to ensure the adhesion of the aggregate to the binder and pavement surface.
When aggregate is spread onto a road surface where there is no binder, the aggregate will not stick to the road, but remains as loose gravel on the road. Of course, such loose gravel can create several problems on a roadway.
When aggregate is laid down where there is no binder, the aggregate tends to be wasted because it is not being used as part of the chip seal as it was intended. Thus, the more aggregate that is spread over surfaces without binder, the more aggregate is wasted during the chip sealing operation.
Aggregate not bound to the road surface by a binder is free to be moved, and sometimes flipped upwards by traffic. When this loose aggregate is flipped upwards by cars it can cause damage to traffic nearby. For example, a piece of aggregate flipped upward could hit and crack a windshield, or chip the paint of the vehicle. Flipped aggregate could also hit and injure a pedestrian.
If a substantial amount of aggregate were placed on the roadway such that it was loose and not bound to the road by binder, car accidents could be caused. For example, if a car hit a large portion of loose aggregate at high speed, it could swerve out of control and collide with oncoming traffic, an embankment, pedestrians, etc.
Often a chip seal cannot be applied to an entire roadway at one time but requires two or more passes. For example, for a two-lane highway, often one lane will be chip sealed, and then the other lane will be chip sealed thereafter. In such cases, the first lane will usually have binder applied to it, and after that the chip spreader travels down that lane applying aggregate. Thereafter, the second lane is chip sealed in similar fashion.
Before the chip sealing process can be applied, the road surface needs to be substantially free of loose aggregate. Sometimes, while spreading aggregate on one lane which has had the binder applied, excess aggregate can be inadvertently spread onto the other lane that does not yet have binder applied. Several factors can contribute to this. The binder may simply have not been applied properly in some areas. Alternatively, at times, chip spreader operators must maneuver the chip spreader in a way that may cause the aggregate to be applied where the binder is not. An example of this is when an aggregate spreader operating in a rural area must steer so as to avoid a mailbox.
If loose aggregate is found on the lane not yet chip sealed, the aggregate must be cleared off before the binder coat can be applied. This requires additional labor. Often the aggregate is removed by manual laborers using sweepers. A mechanized sweeper could also be used. In any event, misapplied aggregate often requires additional cost and time in cleaning up the excess or misapplied aggregate.
Not only are there problems when aggregate is applied where there is no binder, but there are problems when no aggregate is applied over binder. Bare binder on a road surface is quite tacky, and often causes vehicles passing over the roadway to have portions of the binder flipped onto them. As many drivers are aware, this binder substance is difficult to get off of a vehicle.
Chip spreaders are complex vehicles to operate. In addition, as discussed, if a chip spreader is not operated and driven correctly, excess aggregate can be spread, causing several possible problems. Because of this, often few members of a road construction crew are qualified to operate a chip spreader. A substantial amount of investment in training and experience is required before an aggregate spreading operator is well qualified.